Heretofore, conventional processing of CMOS, BiCMOS, and other MOS devices had many complex steps. Many of these complex steps are related to the selective implantation of n and p type tanks. One example of such a process flow is shown in FIG. 1.
An oxide layer is grown over a semiconductor substrate. A nitride layer is deposited, patterned, and etched to form a pattern for the n-tanks. After the implantation of the n-type dopant, oxide is grown in the regions devoid of nitride. Once the oxide is grown, the nitride is removed and the p-type regions are formed. Because the oxide is thicker over the regions doped by n-type dopant, the p-type dopants do not penetrate the oxide and diffuse into the n-tanks. Next, the oxide is removed.
The process of FIG. 1 creates an undesirable step in the n-tank as illustrated in FIG. 2. Because a thicker oxide was grown over the n-tanks and growing oxide over silicon consumes a portion of the silicon to form silicon dioxide, a significant step is formed. The step in the silicon in the n-tank causes serious performance degradations and makes processing more difficult.
The prior art saves a masking step, p-tank, by using a self-aligned process as described above. However, for a complex process with multiple tanks, such as low-voltage and high-voltage tanks, there is no savings in the number of mask steps and, in fact, an additional plasma etch may be required.
It is an object of this invention to eliminate some of the process steps for defining the n and p-tanks. In conjunction with the desire to eliminate process steps, it is also desirable to keep the device as planar as possible. Another object of this invention is to eliminate the step in silicon in the n-tank regions.
Other objects of the present invention are to mask and implant tanks of different concentration or conductivity type without having to anneal after each implantation step; and perform a majority of the high-temperature process steps early in the process flow. Further objects and advantages of the invention will become apparent to those of ordinary skill in the art having reference to the following specification together with the drawings.